The present invention relates to a submarine telecommunication line comprising optical fibers, which is formed by sections of submarine cables of the optical fiber type arranged in series with one another and between which opto-electronic repeaters of the transmitted signals are interposed.
As is known, in any submarine telecommunication line comprising optical fibers, the presence of a structure which is mechanically resistant to the tension stresses exerted on the line during its laying or during its recovery for possible repairs is essential. As used therein, "mechanically resistant" means that the identified element has a strength sufficient to withstand the identified stress or stresses. The components of this mechanically resistant structure of a submarine line are constituted by the tension resistant armors of the cables and of the opto-electronic repeaters.
The tension resistant armor for optical fiber submarine cables can be divided into two groups.
A first group of optical fiber submarine cables comprises those cables in which an armor, mechanically resistant both to the tension stresses imparted to it during laying and to the stresses due to the hydrostatic pressure on the laid cables, is arranged around the cable portion occupied by the optical fibers. In other words, the outer armor has a mechanical strength sufficient to withstand the stresses to which the cable is subjected.
A second group of optical fiber submarine cables comprises cables in which the mechanically resistant armor is surrounded by the optical fibers. In particular, in cables of this kind, the armor is only a rope which occupies the radially innermost portion of the cables. Moreover, they are provided only with a water-tight sheath, substantially devoid of any mechanical resistance to the stresses due to the hydrostatic pressure, since the resistance to the latter is obtained by filling any space inside the sheath which is free of other material with an incompressible fluid. See, for example, U.S. Pat. No. 4,690,498.
Also, the opto-electronic repeaters can be divided into two groups.
A first group comprises opto-electronic repeaters, the structure of which is such as to allow the anchoring to them only those cables in which the mechanically resistant armor surrounds the optical fibers.
In the opto-electronic repeaters belonging to this first group, the mechanically resistant structure is constituted by a water-tight envelope or metal casing which is provided at its two ends with elements for fastening the armors of two cables of the first group to which the repeater is connected.
The repeaters belonging to the first group have a simple and compact structure and have a resistance to damage which might be caused by impact with submerged bodies, such as, fishing nets and the like.
In fact, the repeaters belonging to the first group are provided with water-tight envelopes or metal casings of a thickness which ensures a high mechanical resistance, such envelopes or casings having a size which will withstand not only hydrostatic pressure but also the tension stresses exerted during the laying of the line.
A second group of opto-electronic repeaters is composed of repeaters having a structure able to allow the anchoring to them only of cables of the second group in which the mechanically resistant armor is constituted by a rope occupying the radially innermost portion and surrounded by the optical fibers.
In the opto-electronic repeaters belonging to the second group, the mechanically resistant structure is constituted by a tie-rod, or the like, which occupies the repeater's radially innermost portion. See, for example, my co-pending U.S. application Ser. No. 875,305 filed June 17, 1986, now U.S. Pat. No. 4,767,173. The opto-electronic components of the repeaters are positioned around the tie-rod and the whole is surrounded by a water-tight envelope except for the ends of the tie-rods which project in cantilever fashion from the ends.
In the opto-electronic repeaters belonging to the second group, each space inside the envelope free from other material is filled with an incompressible fluid, so that said envelope, by itself, has a small mechanical resistance. Consequently, these repeaters are likely to be damaged by impacts.
From the foregoing, it can be understood that the conventional submarine telecommunication lines comprising optical fibers are normally formed entirely by cables of the same kind, connected together by opto-electronic repeaters of the corresponding group.
In particular, said conventional lines are formed only by cables of the first kind connected together by opto-electronic repeaters belonging to the first group or only by cables of the second kind connected together by opto-electronic repeaters of the second group.
The already known submarine telecommunication lines comprising optical fibers have, at the same time, both advantages and drawbacks which will be considered hereinafter.
The already known submarine lines formed by cables of the first kind connected together by opto-electronic repeaters of the first group are extremely heavy, relatively inflexible and difficult to handle, and this causes difficulties in carrying out the laying operations.
Further, when the laying depth becomes greater, the weight of the line increases and its flexibility and handling are considerably reduced. In fact, an increased laying depth requires a corresponding increase in the size of the armor which, in the cables belonging to the first type, surrounds the cable portion occupied by the optical fibers because the tension stresses exerted on the line during its laying and the stresses due to the hydrostatic pressure applied to the cables are greater.
The same considerations can be applied to the opto-electronic repeaters belonging to the first group since the thickness of their mechanically resistant envelope is increased in accordance with the increase of the laying depth.
An advantage of the conventional submarine lines formed by cables of the first kind is that they are sufficiently resistant to the impacts due to fishing nets, and the like, by virtue of a substantial armor which surrounds the part of the line occupied by the transmission components.
However, the resistance to impacts is advantageous only in the submarine cable lengths laid in proximity to shores where the risk of impacts is high, whereas, as a practical matter, said risk does not exist for the remaining part of the line (greater line length by far) since it is laid in very deep areas where no substantial activity is expected. Consequently, with lines of the first kind, the above-described drawbacks in the laying operation is not eliminated.
The conventional submarine lines formed by cables of the second kind connected together by opto-electronic repeaters of the second group are, instead, light, flexible and easy to handle, and this facilitates the laying steps.
However, an inconvenience of these lines is that they can be easily damaged in the event of impacts against submarine bodies, such as, for example, fishing nets and the like.